1. Field of the Invention
The present invention relates to a pipe laying vessel and more particularly to a vessel capable of constructing a pipeline from pipe segments and laying it offshore in a number of different modes. The invention also relates to the use of such a vessel in the construction of a pipeline.
2. Description of the Related Art
Underwater pipes, cables and other structures must frequently be provided for various industries. For the oil and gas industry, pipelines may be required between rigs and wells, for transport onshore or even for transport across a body of water from one country to another. Flexible pipe is available which can be provided on a roll in relatively long segments and deployed overboard. Nevertheless, for large bore pipelines, the pipeline is constructed from individual pipe segments which must be welded together during the pipe laying procedure to form the pipeline. Such work is frequently performed using pipe-laying vessels equipped to fabricate the pipeline on board the vessel itself, and to lay the pipeline as it is fabricated. The present invention is directed to such constructed pipelines.
There are two main principles of operation for deploying constructed pipeline, generally referred to as S-lay and J-lay methods. S-laying refers to methods of fabricating the pipeline on a substantially horizontal assembly line on board the vessel, and launching the pipeline overboard via a curved laying ramp otherwise referred to as a stinger. The path of the pipeline from the vessel, through the water and along the seabed describes an S-shape. J-laying refers to a method, whereby the pipeline is deployed downwards from a substantially vertical tower. As the pipeline is lowered it adopts a J shape between the vessel and the seabed. S-laying is the preferred procedure for most situations, since it allows for greater speed of construction due to the efficient and near continuous operation of the assembly line. The J-lay procedure is generally used when working in deep water, where the weight of suspended pipe might otherwise impose excessive stress in the first overbend at the point of departure from an S-lay stinger. The J-lay procedure is considerably slower, since the operation is intermittent with each new segment of pipe being welded to the string before advancing the pipeline. Other situations may dictate use of a J-lay procedure, in particular when working close to a rig or other object.
Dedicated vessels have been developed for performing each of these procedures in an optimised manner. There are also a number of vessels that attempt to perform both procedures. One such vessel is described in WO2011010207 which provides for a preassembly line below deck for joining pipes into pipe assemblies, capable of deploying the pipeline in an S-lay mode and which also includes a J-lay tower, capable of deploying pipeline in a J-lay mode over a side of the vessel. To this end, the J-lay tower is mounted to an outboard balcony structure which adds considerable width to the vessel. The outboard position of the J-lay tower also makes it vulnerable to rolling motion of the vessel in high seas.
Another vessel is described in WO2012/101233 which has an S-lay assembly including a firing line extending along the length of the vessel and a J-lay tower located above the firing line. The vessel has an extended stern section within which an S-lay opening is provided. Combining J-lay and S-lay apparatus in a single vessel allows for operation over a multitude of depths and situations. Nevertheless, the provision of both facilities within the confines of a single vessel requires compromise, especially if it is also to provide additional facilities such as heavy lifting cranes. Additional length significantly increases the cost of a vessel. Height and width are also limited if the vessel is to pass through locks and under bridges.
It would therefore be desirable to provide a vessel which is configured to provide J-lay and S-lay operations without significantly increasing its dimensions.